1. Field of Invention
The present invention relates to a method for forming a nitride. More particularly, the present invention relates to a method for forming a nitride on a silicon substrate.
2. Description of Related Art
The silicon substrate has the properties including low price, large wafer size and high thermal and electrical conductivities and the potential for integrating with electrical circuit so that the silicon substrates attract a lot of attentions and are widely used in the epitaxy technology. For instance, the silicon substrate is used as the substrate for the growth of the nitride semiconductor device.
However, the thermal expansion coefficient of the silicon substrate is different from that of the film made by the epitaxy (taking the nitride-containing semiconductor device as an example). Thus, a tensile stress is generated within the nitride during the temperature dropping process after the nitride is formed and the tensile stress pulls and drags the lattice structure of the nitride so as to form cracks in the nitride. Therefore, how to form a crack-free epitaxy structure on the silicon substrate becomes one of the important issues to one ordinary skilled in the art.
Currently, in the conventional technology, trenches are formed on the silicon substrate to release the stress in the nitride. However, the aforementioned method limits the variations of the device layout. Moreover, another conventional technology uses a buffer layer structure formed between the nitride and the silicon substrate to decrease the negative effect from the thermal stress. For instance, in the formation of the ternary aluminum gallium nitride buffer layer, by step-by-step modulating the ratio of aluminum to gallium, a compressive stress is generated to compensate the tensile stress generated within the nitride during the temperature dropping process after the nitride is formed so as to decrease the negative effect from the thermal stress and further to obtain the crack-free quality of the epitaxy. However, steps of the aforementioned method are complex.